Method and apparatus for computing sir of time varying signals in a wireless communication system

ABSTRACT

A method and apparatus for correcting symbols of a common pilot channel (CPICH) to generate an accurate signal-to-interference ratio (SIR) estimate in a wireless communication system are disclosed. In one embodiment, a non-stationary mean of a group of the CPICH symbols is estimated, the CPICH symbols are delayed, and the CPICH symbols are corrected by dividing the delayed CPICH symbols by the estimated non-stationary mean of the group of CPICH symbols. In another embodiment, a signal power estimate is generated based on the magnitude of CPICH symbols, a noise power estimate is generated by subtracting the signal power estimate from a total power estimate based on the magnitude of the CPICH symbols, and a SIR estimate is generated for symbols that have undergone a time varying gain by dividing the signal power estimate by the noise power estimate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/191,533, filed Aug. 14, 2008, which is a continuation of U.S. patentapplication Ser. No. 11/302,819, filed Dec. 14,2005, which issued asU.S. Pat. No. 7,421,045 on Sep. 2, 2008, which claims the benefit ofU.S. Provisional Patent Application No. 60/663,321, filed Mar. 18, 2005,which are incorporated by reference as if fully set forth.

FIELD OF INVENTION

This application is related to wireless communications.

BACKGROUND

Advanced wireless communication systems, such as third generation (3G)high speed downlink packet access (HSPDA) systems, often transmit acommon pilot channel (CPICH) signal that is used to estimate the qualityof a communication channel. One of the typical methods that are used toestimate channel quality includes estimating the SIR of a pilot signal.In packet based systems, the received power level may change often aspackets are transmitted at different power levels. An automatic gaincontrol (AGC) circuit is used at a receiver in a wirelesstransmit/receive unit (WTRU) (i.e., a mobile station) to react to thechanges in the power level by adjusting the gain.

In a conventional wireless communication system which generates a pilotsignal, time varying gain changes occur on the pilot signal. Thus, thepilot signal becomes non-stationary which results in making it moredifficult to estimate the true SIR of the signal. Typically, the trueSIR will be underestimated. When the measured SIR is lower than theactual SIR is indicated to the system that the channel quality is worsethan it actually is, thus leading to lower throughput and inefficientuse of radio resources.

When a 3G HSDPA system has large total transmit (Tx) power variations,as is possible when some subframes carry packets while other subframesdo not, the AGC circuit will adapt to the new power level at a rate thatdepends on implementation of the AGC circuit, while the transmittedpilot power has not changed. Therefore, the pilot symbols which arecomputed by despreading pilot chips will have mean values and variancesthat change with time, causing the AGC gain to change. This distorts theaccuracy of the SIR computation, as shown in FIG. 1. Because the gainvaries during the SIR measurement period, the SIR estimate is ofteninaccurate when compared to true SIR.

SUMMARY

The present invention is related to a method and apparatus forcorrecting symbols of a CPICH to generate an accurate SIR estimate in awireless communication system. In one embodiment, a non-stationary meanof a group of the CPICH symbols is estimated, the CPICH symbols aredelayed, and the CPICH symbols are corrected by dividing the delayedCPICH symbols by the estimated non-stationary mean of the group of CPICHsymbols. In another embodiment, a signal power estimate is generatedbased on the magnitude of CPICH symbols, a noise power estimate isgenerated by subtracting the signal power estimate from a total powerestimate based on the magnitude of the CPICH symbols, and a SIR estimateis generated for symbols that have undergone a time varying gain bydividing the signal power estimate by the noise power estimate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawingwherein:

FIG. 1 shows an example of CPICH symbol amplitudes after a discontinuouspower level change due to AGC in a conventional wireless communicationsystem;

FIG. 2 is a block diagram of an apparatus including a CPICH symbolcorrection unit for correcting CPICH symbols prior to performing a SIRestimate in accordance with the present invention;

FIG. 3 is a flow diagram of a piece-wise linear curve fitting processperformed by a gain versus time estimator in the CPICH symbol correctionunit of FIG. 2;

FIG. 4 shows gain correction of symbols as implemented by the apparatusof FIG. 2; and

FIG. 5 is a block diagram of an exemplary apparatus for performing SIRestimates using a first order curve fitting procedure on groups of 10symbols to implicitly correct symbols for SIR estimation in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides more accurate SIR computations of signalsthat have time varying statistics due to application of a time varyinggain, i.e., are non-stationary. The channel quality is computed moreaccurately and thereby increases the system and WTRU throughput. Themethod of the invention estimates the effect of AGC or other adaptivecomponents on the symbols. The estimated effect of such components onthe symbols is taken into account when the SIR is calculated, thusresulting in a more accurate estimate of the SIR.

The features of the present invention may be incorporated into anintegrated circuit (IC) or be configured in a circuit comprising amultitude of interconnecting components.

In accordance with the present invention, the symbols may first becorrected in order to compute the SIR or alternatively the correctioncan be incorporated into the SIR algorithm. In a preferred embodiment,the method of the invention performs SIR measurements by removing theeffects of the time varying gain. FIG. 2 shows a top-level diagram thatillustrates the performing of the SIR measurements by removing theeffects of the gain variations. The CPICH symbol amplitudes arecollected, and a time history of the symbols is used to estimate thegain variations during that history. The estimated gain history (gainversus time estimate) is used to correct the corresponding symbols andtherefore the symbol amplitudes. Once corrected, the symbols may beprocessed with conventional SIR algorithms.

FIG. 2 is a block diagram of an apparatus including a CPICH symbolcorrection unit 200 for correcting CPICH symbols prior to performing aSIR estimate using a SIR estimator 205 in accordance with the presentinvention. The CPICH symbol correction unit includes an optionalmagnitude unit 210, a delay unit 215, a gain versus time estimator 220and a divider 225.

Referring to FIG. 2, it is assumed that the CPICH symbols 230 aredespread CPICH symbols 230. However, it should be understood by one ofordinary skill in the art that the input to the CPICH symbol correctionunit 200 may be any other type of phase modulated symbols. The magnitudeunit 210 is optional and is useful for reducing complexity of the CPICHsymbol correction unit 200. Each CPICH symbol 230 is a complex number.The output of the magnitude unit 210 is the magnitude of the inputcomplex numbers associated with the CPICH symbols 230, referred tohereinafter as the CPICH magnitude signal 235. Thus, the CPICH magnitudesignal 235 is equal to the magnitude of the CPICH symbols 230. Sincechanging the phase of a complex number does not alter its magnitude,phase noise and phase modulation do not impact the SIR estimation if themagnitude unit 210 is used. The magnitude unit 210 may also be used tosuppress the impact of phase noise on a SIR estimate 255 generated bythe SIR estimator 205. The CPICH magnitude signal 235 is fed to both thedelay unit 215 and the gain versus time estimator 220.

The gain versus time estimator 220 estimates the non-stationary, ortime-varying, mean of a group of symbols included in the signal 235, asshown in FIG. 1, where the input symbols of signal 235 are representedby the open circles and the non-stationary time-varying gain that theinput symbols are subjected to is represented by a solid line. The gainversus time estimator 220 outputs a signal 245, which is an estimate ofthe non-stationary time-varying gain shown in FIG. 1. The delay unit 215generates an output 240 which is a delayed version of the signal 235.The delay unit 215 is used so that the gain versus time estimator 220has sufficient time to collect a set of symbols from the signal 235,estimate the non-stationary mean of the symbols, and generate the signal245 such that it is time-aligned with the signal 240. Since it takessome time to compute the time-varying mean of the CPICH symbols 230, andit is desired to divide each symbol by its mean, the symbols are delayedsuch that when symbol X is on the output 240 of the delay unit 215, theestimated mean of symbol X is also on the output 245 of the gain versustime estimator 220. The divider 225 divides each symbol on output 240 byan estimate of its mean on output 245, thereby removing the gain errorand providing corrected symbols on output 250. The corrected symbols canthen be used as the input to the SIR estimator 205 under the assumptionthat the symbols are stationary, which indicates that the statistics ofthe symbols, e.g., the mean, do not change with time. By dividing eachsymbol by its mean, set of symbols is created that have the same mean,thus effectively removing the time varying gain. Therefore, the SIRestimate 255 generated by the SIR estimator 205 more accurately measuresthe true SIR of the despread CPICH symbols 230.

Any number of curve-fitting filtering methods may be used, as is knownto one of ordinary skill in the art. In accordance with the presentinvention, a preferred curve-fitting filtering method using piece-wiselinear estimation on each group of symbols may be implemented by thegain versus time estimator 220, thereby creating a linear curve segmentfor each group. The straight line segments shown in FIG. 4 illustratethe piece-wise linear curve generated by the gain versus time estimator220.

FIG. 3 is a flow diagram of a piece-wise linear curve fitting process300 performed by the gain versus time estimator 220 in the CPICH symbolcorrection unit 200 of FIG. 2. For example, FIG. 4 shows a gaincorrection curve with three linear segments of estimated gain shown.

Referring to FIG. 3, in step 305, the input symbols are partitioned intoM groups of symbols. The size the groups may all be the same size but ingeneral each group size L(m), m=1 . . . M, may have a different size.For each group m, step 310 defines two vectors. The vector Y is formedfrom the elements (i.e., symbols) in a current group of symbols, and thevector X is just the sequence of numbers from —[L(m)−1]/2 up to[L(m)−1]/2 in steps of one. In step 315, the variable A is calculatedbased on the average of Y. This is the average value of the best fitstraight line through the symbols. In step 320, the variable B iscalculated by dividing the vector inner-product of X and Y by theaverage of the square of the elements (i.e., symbols) in X, B32(XTY)/average(X²). This is the slope of the best fit straight linethrough the symbols. In step 325, the best fit straight line, C_(m),though the symbols in group m are computed. In step 330, m is comparedto M to determine if all groups have to be accounted for. If m<M, m isincremented in step 335 and the process is repeated starting at step310. If m=M, the set of Cm's are concatenated in step 340 and outputfrom the gain versus time estimator 220 as signal 245.

FIG. 4 shows the correction of symbols that have been subjected to atime varying gain using the apparatus 200 and process 300 illustrated inFIGS. 2 and 3, respectively. The solid line shows the original gainerror (or equivalently the mean) of the input symbols. FIG. 1 also showsthe original gain error of the input symbols and also the input symbolsthemselves shown as open circles.

Still referring to FIG. 4, the dashed line is the estimated time-varyingmean of the input symbols generated by gain versus time estimator 220.Note that this curve is composed of 3 straight sections. The inputsymbols are divided by this curve to produce the corrected symbols shownas open circles in FIG. 4. Note the difference between the open circlesin FIG. 4 and FIG. 1. The residual gain, or the effective gain aftercorrection of the symbols, is shown as the dotted line in FIG. 4.

FIG. 5 is a block diagram of an exemplary apparatus 500 for performingSIR estimates using a first order curve fitting procedure on groups of10 symbols to implicitly correct symbols for SIR estimation inaccordance with another embodiment of the present invention. Theapparatus 500 includes a serial-to-parallel (S/P) converter 505, a curvefitting filter 508, a magnitude squared unit 545, an accumulator 550, anadder 555, a signal power filter 560, a noise power filter 565 and adivider 570. The curve fitting filter 508 includes accumulators 510,530, magnitude squared units 515, 535, a multiplier 520, an element-wisevector 525 and an adder 540.

In this example, a plurality of magnitude CPICH symbols 502 are groupedinto a vector often (10) symbols 504 by an S/P converter 505. The symbolvector 504 is fed to the curve fitting filter 508 and the magnitudesquared unit 545 such that the apparatus may simultaneously correct andcompute SIR of the symbols. In the curve fitting filter 508, theaccumulator 510 and the element-wise vector multiplier 525 receive thesymbol vector 504.

The accumulator 510 adds all of the symbols in the symbol vector 504 toproduce a sum 506 which is squared by the magnitude squared unit 515 toproduce a magnitude squared sum 509, which is multiplied by a referencesignal 511 having an amplitude equivalent to one divided by the numberof symbols in the vector 504 (which is 0.1 for this example) by themultiplier 520 to produce a product signal 513.

The element-wise vector multiplier 525 multiplies the vector signal 504by a constant sequence vector 514 to produce a vector signal 516. Theelements (i.e., symbols) of the vector signal 516 are summed in theaccumulator 530 to produce a sum 518 which is squared by the magnitudesquared unit 535 to produce a magnitude squared sum 522 which is addedto the product signal 513 by the adder 540 to produce a signal power(P_(s)) estimate 524.

The vector signal 504 is element-wise squared by the magnitude squaredunit 545 to produce a vector signal 528, the elements (i.e., symbols) ofwhich are summed by the accumulator 550 to produce a total powerestimate 532. The signal power estimate 524 is subtracted from the totalpower estimate 532 by the adder 555 to produce a noise power (Pn)estimate 534.

The signal power estimate 524 is filtered by the signal power filter 560to produce a filtered signal power estimate 526. The noise powerestimate 534 is filtered by the noise power filter 565 to produce afiltered noise power estimate 536. The divider 570 computes the ratio ofsignal 526 and signal 536 to produce a SIR estimate 538 for symbols thathave undergone a time varying gain.

In accordance with the present invention, a group of symbols iscollected and used to compute the SIR. The group is partitioned intoseveral smaller groups. Within each smaller group, an Nth order fit(N=0,1, . . .) is computed for the symbol amplitudes and each symbol isdivided by the corresponding value computed by the fit. The SIR can thenbe computed in a standard way.

This present invention is advantageous when gain changes occur butgenerates a small positive bias when no actual gain change occurs. Assuch, the correction algorithm may optionally not be run if it can bedetermined that the power level is not changing substantially. Theprocedure is illustrated in FIG. 3 for first order fits (N=1). The pre-and post-gain corrected symbols and computed SIRs are shown in FIG. 1and FIG. 4 respectively, using a partition into 3 groups and using 1storder fits (N=1).

In another embodiment of the invention, the invention makes use ofapriori knowledge of both the time of the power level change and thecharacteristics of the AGC. In this case, the group is not partitioned.Instead, the size of the gain change is estimated (either by measurementof symbols or directly from gain control stage). Since thecharacteristics of the AGC are known, the gain as a function of timethroughout the group of symbols is easily computed and used to dividethe corresponding symbols' amplitudes. If the location of the powerlevel change is not known apriori, it may also be estimated. The AGCcharacteristics can also be estimated if they are not known.

As an example, a linear (first-order) curve fitting approach is used foreach group of CPICH symbols collected. Coefficients for the first-ordercurve fitting polynomial are attained by least squares, which minimizethe sum of squared residuals, or

$\begin{matrix}{\min {\sum\limits_{i = 1}^{N}\left( {y_{i} - {\hat{y}}_{i}} \right)^{2}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where N is the number of observed CPICH symbols, yi is the receivedsignal, and

ŷ _(i) =b ₀ +b _(l) t _(i)   Equation (2)

is the estimated value with t_(i) as the reference sequence for curvefitting.

The coefficient b₀ and b₁ are easily found taking the partial derivativeof Equation (1) and setting it to zero. The first-order solutions of b₀and b₁ are

$\begin{matrix}{b_{0} = {\overset{\_}{y} - {b_{1}\overset{\_}{t}}}} & {{Equation}\mspace{14mu} (3)} \\{{b_{1} = \frac{{\sum\limits_{i = 1}^{N}{y_{i}t_{i}}} - \; {N\overset{\_}{y}\overset{\_}{t}}}{{\sum\limits_{i = 1}^{N}t_{i}^{2}} - {N\overset{\_}{t}}}}{where}{\overset{\_}{y} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}y_{i}}}}{and}{\overset{\_}{t} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}{t_{i}.}}}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

A zero-mean sequence is used so that, t=0, which simplifies the aboveequations. Also note that

$\frac{1}{\sum\limits_{i = 1}^{N}t_{i}^{2}}$

is a constant and can be replaced with a constant gain K.

$\begin{matrix}{b_{0} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}y_{i}}}} & {{Equation}\mspace{14mu} (5)} \\{b_{1} = {\frac{1}{K}{\sum\limits_{i = 1}^{N}{y_{i}t_{i}}}}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

With the available curve fitting coefficients, the following equationsmay be used to define the estimates of signal power and noise power andthus the SIR estimate. The power of the CPICH magnitude is preferablyestimated by

$\begin{matrix}\begin{matrix}{P_{s} \approx {\sum\limits_{i = 1}^{N}\left( {b_{0} + {b_{1}t_{i}}} \right)^{2}}} \\{= {{\sum\limits_{i = 1}^{N}b_{0}^{2}} + {\sum\limits_{i = 1}^{N}{b_{1}^{2}t_{i}^{2}}}}} \\{= {{Nb}_{0}^{2} + {b_{1}^{2}K}}}\end{matrix} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

where b₀ and b₁ are obtained by passing the CPICH magnitude (M_(i))through the curve fitting filter. Substituting b₀ and b₁ into the aboveequation leads to

$\begin{matrix}{P_{s} = {{\frac{1}{N}\left( {\sum\limits_{i = 1}^{N}M_{i}} \right)^{2}} + {\frac{1}{K}\left( {\sum\limits_{i = 1}^{N}{M_{i}t_{i}}} \right)^{2}}}} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

The power of the CPICH signal-plus-noise magnitude is

$\begin{matrix}{{P_{s} + P_{n}} = {\sum\limits_{i = 1}^{N}M_{i}^{2}}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

Therefore, the CPICH magnitude SIR is preferably given by

$\begin{matrix}{{SIR} = {\frac{P_{s}}{P_{n}} = \frac{P_{s}}{{\sum\limits_{i = 1}^{N}M_{i}^{2}} - P_{s}}}} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

As shown in FIG. 5, optional filtering of the numerator and denominatoris implemented to decrease the variance of the SIR estimate. The numberof observed symbols N is chosen to be 10. The constant zero-meanreference sequence for curve fitting is chosen to be t=[−4.5, −3.5,−2.5, −1.5, −0.5, 0.5, 1.5, 2.5, 3.5, 4.5]^(T). In this case, K=82.5 canbe combined with t. Thus the constant sequence vector becomes t/√{squareroot over (82.5)} as shown.

Although the embodiments are described in the context of a 3G HSDPAsystem, the invention applies generally to any packet based system suchas IEEE 802 standard. Although the features and elements of the presentinvention are described in the preferred embodiments in particularcombinations, each feature or element can be used alone without theother features and elements of the preferred embodiments or in variouscombinations with or without other features and elements of the presentinvention.

1. A method for generating a signal to interference ratio estimate inwireless communications, comprising: collecting common pilot channel(CPICH) symbols and associated symbol amplitudes; estimating gainvariations based on a time history of the CPICH symbol amplitudes;generating an estimated gain history by relating gain to a timeestimate; and correcting corresponding symbols and associated symbolamplitudes based on the estimated gain history.
 2. The method of claim1, wherein the CPICH symbols are despread CPICH symbols.
 3. The methodof claim 1, wherein the CPICH symbols are any other type of phasemodulated symbols.
 4. The method of claim 1, wherein each CPICH symbolis a complex number.
 5. The method of claim 1, further comprising:generating a magnitude signal equal to the magnitude of the CPICHsymbols.
 6. A symbol correction unit configured to collect common pilotchannel (CPICH) symbols and associated symbol amplitudes, comprising: again versus time estimator configured to: estimate gain variations basedon a time history of the CPICH symbol amplitudes; and generate anestimated gain history by relating gain to a time estimate; and adivider configured to correct corresponding symbols and associatedsymbol amplitudes based on the estimated gain history.
 7. The symbolcorrection unit of claim 6, further comprising: a magnitude unitconfigured to generate a magnitude signal equal to the magnitude of theCPICH symbols.
 8. The symbol correction unit of claim 6, wherein theCPICH symbols are despread CPICH symbols.
 9. The symbol correction unitof claim 6, wherein the CPICH symbols are any other type of phasemodulated symbols.
 10. The symbol correction unit of claim 6, whereineach CPICH symbol is a complex number.